1. Filed of the Invention
The present invention relates to an attachment structure of an antenna unit that receives radio waves sent from, for example, a satellite onto a vehicle. Particularly, the invention relates to improvement of an attachment structure onto a plastic attachment part.
2. Description of the Related Art
Heretofore, a system which navigates a car route for a driver of an automobile, that is, a car navigation system, has been widely applied. In such a car navigation system, a present position is specified according to speed and running distance of a car. In order to improve positional accuracy, radio waves that include position data are sent from a GPS satellite to the car navigation system.
Alternatively, the USA has recently begun providing digital radio broadcasting with use of radio waves sent from an artificial satellite. For a digital radio reception system that receives digital radio broadcasting, an antenna is necessary for receiving radio waves sent from the satellite, and a so-called satellite radio antenna has been used.
Since the radio wave from the satellite uses a high-frequency band, the satellite has high directivity. Therefore, for the car navigation system and the digital radio reception system, it is necessary to attach a reception antenna on an upper surface (that is, a roof or trunk) of a car in order to have good reception when radio waves are received.
When attaching the antenna unit, method of boring a hole in, for example, a vehicle roof to mount the antenna unit and fixing with a nut and bolt has been widely applied. If the roof is a metal plate, a waterproof countermeasure is taken by interposing a rubber gasket between a vehicle roof and an antenna base.
For instance, the example of the related art includes JP-A-8-84207.
In a vehicle such as automobile, weight reduction has been advanced, so that plastic members are used in a roof, a bonnet, trunk, and the like.
If the above-mentioned antenna unit is attached onto such plastic members, a problem of ground (GND) occurs. Using the satellite radio antenna as an example, if an antenna having GND dependency, such as a patch antenna, a loop antenna or the like, is mounted on the vehicle, it utilizes a roof or a trunk that is composed of metal as a ground (GND) plate. However, if the roof or trunk is composed of plastic, the ground of the antenna cannot be secured.
In order to solve this problem, a progressive wave type antenna or an antenna that includes a ground plate is used so that the ground can be secured by the antenna itself. However, these prior methods result in significant increases in the height and area of the antenna, so that these antennas are not preferable in appearance. Further, these antennas are expensive, so that they are disadvantageous also from a viewpoint of cost.
Further, there are problems in the prior attachment methods. If the antenna unit is attached onto the roof or trunk composed of a metal plate, a method of boring a hole in the roof or trunk, mounting the antenna unit in this hole, and fixing the antenna unit with a bolt and a nut has been applied. If the material of the roof or trunk is plastic (resin), the material cannot correspond to bolt tightening power in the bolt type method.